In the related art, a thrust bearing disposed opposite to a thrust collar installed at a rotary shaft is well known as a bearing for a high speed rotor. Among thrust bearings, for example, a thrust dynamic pressure bearing using a dynamic pressure effect has a spiral groove formed at a bearing surface of the bearing, and as a fluid lubricating film is formed between the thrust collar and the bearing surface, the rotary shaft is supported via the lubricating film.
A pump-in type (the structure type), a pump-out type, and a herringbone type are well known as spiral grooves of the thrust bearing, which are formed in the bearing surface of the thrust dynamic pressure bearing. In the spiral groove, for example, in the pump-in type or pump-out type, a plurality of spiral grooves (spiral-shaped grooves) are formed at the same pitch. All of the spiral grooves are formed to have the same inflow angle. One end of the spiral groove is closed by a land section in the innermost circumference or the outermost circumference of the bearing surface, and the other end is opened.
A type (a structure type) in which the land section is formed at the innermost circumference is referred to as the pump-in type, and a type in which the land section is formed at the outermost circumference is referred to as the pump-out type. When these are assembled to the thrust collar, which is rapidly rotated, a lubricating fluid sandwiched between the thrust collar and the bearing by rotation of the thrust collar is pulled to the thrust collar through viscosity thereof. As a result, the lubricating fluid flows along the groove. For example, in the case of the pump-in type, the lubricating fluid is drawn at the outermost circumference of the bearing surface. The drawn lubricating fluid is introduced (drawn) to the innermost circumferential side from the outermost circumference of the bearing along the groove. After that, the lubricating fluid runs on the innermost circumference land section at the closing end of the groove. Here, as a flow path is abruptly narrowed, a pressure (a film pressure) of the fluid lubricating film is increased.
As a result, the film pressure of the fluid lubricating film of the pump-in type becomes a mountain-shaped distribution in which a peak is formed at a boundary section (the closing end of the groove) between the innermost circumference land section and the groove. Similarly, the film pressure of fluid lubricating film of the pump-out type becomes a mountain-shaped distribution in which a peak is formed at a boundary section (the closing end of the groove) between the outermost circumference land section and the groove. In reality, all the lubricating fluid does not flow in the groove. Some of the lubricating fluid flows in a circumferential direction and runs on a spiral-shaped land section formed between groove rows. For this reason, in the pump-in type, the film pressure is also generated between the spiral groove of the outer circumferential side and the land section (between the groove closer to the outer circumferential and the land section). In the pump-out type, the film pressure is also generated between the spiral groove of the inner circumferential side and the land section (between the groove closer to the inner circumferential and the land section).
In addition to the pump-in type or the pump-out type, a bearing having a spiral groove in which the herringbone type, pump-in type, and pump-out type are assembled is well known (for example, see Patent Document 1 and Patent Document 2).